The present invention relates to power transmitting fluids, particularly automatic transmission fluids, of enhanced performance capabilities.
Automobile builders continually strive to produce more durable vehicles. Overall vehicle durability is a function of the durability of each of the components in the vehicle, e.g., the engine, transmission, etc. In order to improve overall vehicle durability, the durability of every major component in the vehicle must be improved. To improve the durability of the automatic transmission, automatic transmission fluids (ATF's) of improved performance must be produced. Relative performance of an automatic transmission fluid can be determined by evaluating the consistency of transmission shifts with increasing time or mileage, the extent to which the transmission has worn, and the extent to which the ATF has oxidized and sheared (lost viscosity). What we have now found is a method of simultaneously improving all of these ATF performance characteristics.
Although many `bench` tests have been devised to evaluate ATF performance, e.g., SAE #2 friction test machines and Vickers vane pump tests, the best assessment of transmission durability is gained from running an actual transmission. Running a transmission in an extremely severe operating regime, under highly controlled conditions, is quite difficult. However, General Motors has devised such a test. It is described in the General Motors DEXRON.RTM.-III specification as the 4L60 Cycling Test (ATF Specification GM-6297M, April 1993, Appendix F). In this test a full automobile driveline (engine and transmission) is loaded by means of an energy absorption dynamometer. The dynamometer is programmed to simulate the inertia of a fully laden vehicle. With the transmission sump temperature held at 135.degree. C., the engine is accelerated from idle to 3400 rpm under full throttle conditions. Each acceleration from idle to 3400 rpm is termed a `cycle`. A standard test consists of 20,000 cycles. This test correlates very well with severe field service. Using this test to evaluate overall transmission durability and fluid degradation, what we have now found, is that ATF's that contain a minor amount of a high viscosity polyalphaolefin as part of the base oil mixture provide unexpected good transmission durability.